1. Field
The field of the present invention relates to novel methods for detecting and products used to detect the presence of conductive materials, including in the presence of non-conductive materials, wherein the non-conductive materials may have dielectric properties. In addition, the methods and products of the invention detect the presence of specific conductive materials even in the presence of other conductive materials nearby.
2. Background
Conductive Thin Films
Conductive materials include metals, electrolytes, and plasmas, as well as certain non-metallics, such as graphite and conductive polymers. Often, conductive materials are formed into a thin film and coated onto glass or ceramics. For example, conductive thin films are often added to windows to save energy by transmitting visible light and, at the same time, reflecting infrared energy. Conductive thin films can even be used to electronically defrost window panes. In addition to saving energy, conductive thin films block ultraviolet rays, provide shatter resistance, and, when appropriately tinted, provide privacy in, for example, a vehicle or other enclosure.
Thin films generally contain a polyester base made up of at least one layer. More often, the base contains multiple polyester layers bonded together. The polyester material serves multiple purposes, such as acting as a sunlight barrier and reflecting heat.
In addition to the polyester layer or layers, conductive thin films used on windows in vehicles, offices, and houses contain other components. For example, a mounting adhesive may be added to the side of the film next to the window glass and the other side may have a scratch-resistant hardcoat. In addition, if dye technology is used to add the thin film to window glass, then the polyester layer contains a dye, which absorbs heat.
An alternative method used to add a conductive thin film to window glass involves deposition technology, which requires vacuum coating and metalizing. Another method, known as sputtering, involves advanced metalizing. Both deposition and sputtering technologies require deposition of a layer of metallic particles onto the polyester base. Yet another method combines metallic particles and dyes to create a hybrid thin film.
There are many different formulations for thin films and these formulations fall into one of two categories—metallic or non-metallic. Because there are non-metallic thin films that appear metallic and metallic thin films that appear non-metallic, a person cannot determine from visual observation whether a thin film is metallic. However, such a determination is necessary in certain areas, such as antenna performance. For example, antennas are often used in vehicles that have tinted windows or in office buildings with tinted windows, wherein the tinting is in the form of a thin film. If the tinting is metallic, it will severely degrade the performance of an antenna. If the tinting is non-metallic, it will have very little effect on the performance of an antenna.
Metal Detection Techniques
Metal detection has been used to discern the location of objects in various situations. Traditional uses include locating metallic objects of interest in soil or sand and detecting weapons at security screening locations. The common methods used in metal detection are the beat frequency oscillator (BFO), very low frequency (VLF), and pulse induction (PI) methods, which all have a broad sensing area.
Beat frequency oscillator (BFO)-based metal detection methods employ two coils, a search coil and a reference coil, which are placed at a distance from one another such that they do not interfere with one another. When metal objects come close enough to the search coil to detune it and change its frequency, the difference between that frequency and the reference coil's frequency creates a tone indicating the presence of metal. Thus, BFO methods rely on a tone that varies depending on a metallic object's size and distance away from the search coil. It is the simplest method, but the least sensitive to specific objects, especially in conductive surroundings. For example, if BFO equipment is used to detect whether metal is incorporated into a thin film coated on a car window, the metal of the car would dominate the response from the BFO equipment and cause a high false detection rate.
The very low frequency (VLF)-based metal detection methods are similar to the BFO methods in theory of operation. The VLF methods rely on co-locating send and receive coils. Although VLF methods offer improved sensitivity and discrimination between metals and dielectrics, when compared to BFO-based methods, they rely on the detuning effect of metallic or dielectric objects on or near the coils, which are operated at relatively low frequencies. Thus, the presence of a nearby large metal object, such as a car, would cause a high false detection rate.
Pulse induction (PI)-based methods use a single coil to transmit and receive frequencies. The coil is charged and then discharged relatively quickly. A receiver “listens” for a reflected ringing signal from metallic objects in the surrounding environment. Again, however, large metallic objects in relatively close proximity to the object being measured would generate a high false detection rate.
These standard methods of metal detection rely on detecting a large change in the parameters of the circuit components when approaching a small metal object, thus they are designed to operate in mostly non-metallic environments to avoid the potential of large metal objects swamping the desired signal and causing a high false detection rate.